Magnesium alloy for precipitation strengthening extrusion and method of manufacturing the same

ABSTRACT

A tin-containing magnesium alloy having superior tensile strength and superior elongation. A method of manufacturing a magnesium alloy includes melting and casting raw materials including an element selected from the group consisting of more than 0 weight % and 14 weight % or less of Sn, more than 0 weight % and 5 weight % or less of Li, more than 0 weight % and 40 weight % or less of Pb, more than 0 weight % and 17 weight % or less of Al, and more than 0 weight % and 5 weight % or less of Zn and a remainder of Mg, subjecting the cast magnesium alloy to solution treatment, subjecting the solution-treated magnesium alloy to aging, and plastically deforming the aged magnesium alloy. The magnesium alloy has second phases uniformly distributed in crystal grains, has a crystal grain size of 10 μm or less, and exhibits both

CROSS-REFERENCES TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2013-0133968 filed on Nov. 6, 2013, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnesium alloy for precipitationstrengthening extrusion, and more particularly to a tin-containingmagnesium alloy having superior mechanical properties, such as tensilestrength, yield strength, and elongation.

2. Description of the Prior Art

For the sake of performance improvement and weight reduction of variousmechanical devices, there have been ongoing studies to make mechanicaldevices, more particularly, various components thereof lightweight. As ametallic material for such weight reduction of components, magnesium(Mg) alloys have the lowest density among currently developed structuralalloys, and exhibit superior properties regarding electromagneticshielding and vibration absorption. Demand for Mg alloys is on the risein various fields, such as transportation machines, industries relatedto portable components, etc.

According to a Mg extrusion process of the related art, a cast productmanufactured through melting and casting is subjected to homogenizationheat treatment before being extruded. In some cases, a precipitationstrengthening Mg alloy is subjected to aging after the extrusion inorder to improve mechanical properties to some extent.

In the related art, plastic deformation materials produced through theextrusion have typically been alloys containing solute contents withinthe solubilities of Mg, such as AZ31. In contrast, recent Mg extrusionmaterial alloys having high strength and high tenacity contain a largeamount of alloying elements added thereto. In the Mg extrusion materialalloys, some crystallized phases that have been created after castingduring homogenization heat treatment subsequent to melting and castingremain intact inside grains or at the grain boundary, causing irregulardistributions of second phases after plastic deformation. Thisconsequently brings adverse effects on the mechanical properties.

In order to overcome the above problem, solution treatment and aging arecarried out before the extrusion to control the distribution and size ofthe second phases that can improve the strength of materials even afterextrusion. The solution treatment is designed with an optimizedtemperature range in which the crystallized phases created after themelting and casting can be re-dissolved into the matrix. With the aging,the distribution of the second phases mentioned as a problem of therelated-art process can be made uniform.

For reference, FIG. 2 schematically illustrates respective steps of arelated-art method of manufacturing a Mg alloy including tin (Sn), andFIG. 3A schematically illustrates the state of the structure of a Mgalloy including Sn manufactured by the method illustrated in FIG. 2.

As illustrated in FIG. 2, according to the related-art method ofmanufacturing a Mg—Sn-based alloy, raw materials, i.e. Mg, Sn, and otheralloying elements, are melted to form molten metal and are subjected tocasting, homogenization, plastic deformation, and annealing.

In order to obtain a Mg alloy having high strength, however, alloyingelements have recently been added above the solubility in order to addhigh strength and high tenacity. That is, after casting, thecrystallized phases of elements added above the solubility exist in theform of second phases, which are stable at room temperature. Therefore,crystallized phases that have been created in the α-Mg matrix remainintact at the homogenization temperature, and in particular, in the caseof a precipitation strengthening-type alloying element, parts ofsupersaturated elements precipitate in the form of precipitates in thehomogenization temperature range. That is, as illustrated in FIG. 3A,the second phases remaining after homogenization or newly createdprecipitation phases mainly remain inside grains, at grain boundaries,or in regions near the grain boundaries and, when subjected to plasticdeformation such as extrusion, have an irregular distribution in aspecific direction (extrusion direction) of the second phases. Inaddition, when dynamic/static recrystallization occurs after plasticdeformation, the rate of recrystallization increases near the secondphase. As a result, a Mg—Sn-based alloy manufactured by the related-artmethod has irregular distributions of second phases, as illustrated inFIG. 3A. In a region having a large distribution of second phases, thesize of grains is as small as several micrometers due to therecrystallization and the pinning effect at the crystal grainboundaries. However, in a region having a small distribution of secondphases, coarse crystal grains of 10 μm or larger are distributed. Thisresults in a problem in that the average size of crystal grainsincreases, which makes the distribution of crystal grain size irregular,thereby worsening mechanical properties.

The information disclosed in the Background of the Invention section isprovided only for better understanding of the background of theinvention and should not be taken as an acknowledgment or any form ofsuggestion that this information forms a prior art that would already beknown to a person skilled in the art.

SUMMARY OF THE INVENTION

Various aspects of the present invention provide a magnesium (Mg) alloyable to overcome the above-mentioned problems of the related-art Mgalloy and have a uniform size of second phases, and a method ofmanufacturing the same.

Also provided is a Mg alloy having a reduced size of second phases andhaving both superior elongation and superior tensile strength and amethod of manufacturing the same.

According to an aspect of the present invention, there is provided a Mgalloy including: an element selected from the group consisting of morethan 0 weight % and 14 weight % or less of Sn, more than 0 weight % and5 weight % or less of Li, more than 0 weight % and 40 weight % or lessof Pb, more than 0 weight % and 17 weight % or less of Al, and more than0 weight % and 5 weight % or less of Zn; and a remainder of Mg, whereina second phase including at least one selected from the group consistingof Mg₂Sn, Mg₂Zn₃, Mg_(47.2)Zn_(36.9)Al_(16.9), Mg₁₇Al₁₂, α-Mg/β-Liphase, and Mg₂Pb is formed in the alloy, the second phase includesprecipitation phases, and, among the precipitation phases constitutingthe second phase, precipitation phases having a size exceeding 10 μm areless than 0.1% of the entire precipitation phases.

The second phase of the Mg alloy is uniformly distributed in entirecrystal grains.

The size of crystal grains of the Mg alloy is preferably substantiallyevenly distributed.

The Mg alloy may be a plastically deformed plate member.

In this case, the plastically deformed plate member may be an extrudedplate member.

According to another aspect of the present invention, there is provideda method of manufacturing a Mg alloy, the method including the followingsteps of: dissolving and casting raw materials including an elementselected from the group consisting of more than 0 weight % and 14 weight% or less of Sn, more than 0 weight % and 5 weight % or less of Li, morethan 0 weight % and 40 weight % or less of Pb, more than 0 weight % and17 weight % or less of Al, and more than 0 weight % and 5 weight % orless of Zn and a remainder of Mg; subjecting the cast Mg alloy tosolution treatment; subjecting the Mg alloy, which has undergonesolution treatment, to aging; and plastically deforming the aged Mgalloy, wherein a second phase comprising at least one selected from thegroup consisting of Mg₂Sn, Mg₂Zn₃, Mg_(47.2)Zn_(36.9)Al_(16.9),Mg₁₇Al₁₂, α-Mg/β-Li phase, and Mg₂Pb is formed in the alloy, the secondphase comprises precipitation phases, and, among the precipitationphases constituting the second phase, precipitation phases having a sizeexceeding 10 μm are less than 0.1% of the entire precipitation phases.

In this case, the plastic deformation is preferably extrusion.

The second phase is uniformly distributed in entire crystal grains.

The size of crystal grains of the Mg alloy is preferably substantiallyevenly distributed.

Meanwhile, the description that the second phase is uniformlydistributed in the entire crystal grains according to the presentinvention should be interpreted relatively. That is, the descriptionthat the second phase is uniformly distributed in the entire crystalgrains does not mean that second phases or precipitation phases areconcentrated at grain boundaries of crystal grains or at specificportions inside the grains, or are concentrated at some crystal grainsand scarcely exist in some crystal grains as illustrated in FIG. 3A, butmeans that second phases or precipitate phases are distributed in almostall crystal grains in the substantially same amount and, even in eachcrystal grain, are not distributed at the grain boundary of the crystalgrain but are evenly distributed inside the entire crystal grain asillustrated in FIG. 3B.

Furthermore, the description that the size of crystal grains of the Mgalloy is substantially evenly distributed is also relative: not allcrystal grains have the same physical size, but small crystal grainshave a size of a number of μm, and large crystal grains have a sizeexceeding 10 μm, as long as all crystal grains have substantially thesame size in terms of metallography, within a range of a number of μm.

The Mg alloy according to the present invention or the Mg alloymanufactured by the method according to the present invention isadvantageous in that second phases are uniformly distributed insidecrystal grains, the size of which is 10 μm or less. Therefore, the Mgalloy according to the present invention has both superior elongationand tensile strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates respective steps of a method ofmanufacturing a Mg alloy according to an exemplary embodiment of thepresent invention;

FIG. 2 schematically illustrates respective steps of a related-artmethod of manufacturing a Mg alloy;

FIG. 3A and FIG. 3B schematically illustrate the state of the structureof Mg alloys, manufactured by the methods illustrated in FIG. 1 and FIG.2, respectively;

FIG. 4 illustrates engineering stress and engineering strain curves ofMg alloys according to an exemplary embodiment of the present inventionand related-art Mg alloys;

FIG. 5 illustrates a relationship between the UTS and elongation of Mgalloys according to an exemplary embodiment of the present invention andrelated-art Mg alloys;

FIG. 6A to FIG. 6C are SEM pictures of Mg alloys at a step aftersolution treatment and aging and before plastic deformation inconnection with manufacturing of Mg alloys of various compositionsaccording to an exemplary embodiment of the present invention;

FIG. 7A and FIG. 7B are SEM pictures of a Mg alloy manufactured by arelated-art method (FIG. 7A) and of a Mg alloy manufactured according toan exemplary embodiment of the present invention (FIG. 7B);

FIG. 8A is a picture taken after the extrusion of a Mg alloy accordingto a related-art method;

FIG. 8B is a picture taken after the extrusion of a Mg alloy that hasbeen formed by an exemplary embodiment according to the presentinvention; and

FIG. 9A and FIG. 9B are TEM pictures of Mg alloys formed by arelated-art method (FIG. 9A) and according to an exemplary embodiment ofthe present invention (FIG. 9B).

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, an exemplary embodiment of a magnesium (Mg) alloy accordingto the present invention and a method of manufacturing the same will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 schematically illustrates respective steps of a method ofmanufacturing a Mg alloy according to an exemplary embodiment of thepresent invention, and FIG. 3B schematically illustrates the state ofthe structure of a Mg alloy including Sn manufactured by the methodillustrated in FIG. 1.

As illustrated in FIG. 1, a Mg alloy including Sn according to anexemplary embodiment of the present invention is subjected to solutiontreatment, instead of related-art homogenization, and is subjectedaging, plastic deformation, and annealing.

That is, as illustrated in FIG. 3B, according to an exemplary embodimentof the present invention, a Mg alloy including Sn obtained by meltingand casting raw materials is subjected to solution treatment to form asupersaturated solid solution. According to the present embodiment, theMg ally creates crystallized phases after casting, and the crystallizedphases can be re-dissolved into the matrix through the solutiontreatment. Thereafter, aging is performed at suitable heat treatmenttime and temperature so that the precipitation phase (Mg₂Sn phase) canhave even distribution inside grains and at grain boundaries.

That is, when homogenization is performed after casting according to theprior art, the second phase, i.e. precipitate phase, exhibits unevendistribution, and the size of recrystallization crystal grains alsoexhibits uneven distribution. When solution treatment and aging areperformed according to the present embodiment, in contrast, the secondphase, i.e. precipitation phase, has even distribution, and the size ofrecrystallization crystal grains can also have even distribution.

In addition, when homogenization is performed according to the priorart, fine precipitates may occur even at homogenization and plasticdeformation temperatures, but coarse crystallized phases formed aftercasting are largely distributed unevenly. According to the presentembodiment, in contrast, a precipitation process occurs by means ofaging before plastic deformation, so that large second phases that havea size of 2 μm or larger trigger undergo nucleation duringrecrystallization throughout the entire alloy, due to the PSN (particlestimulated nucleation) effect. Precipitate phases generated duringaging, second phases that grow slowly. Small precipitates that have beengenerated during plastic deformation and since grown with a size ofabout 2 μm or less are distributed at grain boundaries after creation ofrecrystallization grains and disturb the growth of grains (pinningeffect). Therefore, the alloy according to the present embodimentsubjected to solution treatment and aging has a big difference regardingthe microstructure, compared with an alloy subjected to related-arthomogenization, and exhibits substantially improved mechanicalproperties.

Results of comparisons between a Mg alloy formed according to anexemplary embodiment of the present invention and a Mg alloy formedusing related-art homogenization using various methods will now bedescribed with reference to FIG. 4 to FIG. 9. Hereinafter, forreference, Mg-5Sn refers to an alloy including 5 weight % of Sn and aremainder of Mg. Mg-5Sn-5Zn refers to an alloy including 5 weight % ofSn, 5 weight % of Zn, and the remainder of Mg. Mg-5Sn-5Zn-2Al refers toan alloy including 5 weight % of Sn, 5 weight % of Zn, 2 weight % of Al,and a remainder of Mg. In addition, Case 1 refers to a Mg alloymanufactured by the related-art method, and Case 2 refers to a Mg alloymanufactured by an exemplary embodiment of the present invention. Forexample, Case1_Mg-5Sn-5Zn refers to a Mg alloy manufactured by therelated-art method. Case1_Mg-5Sn-5Zn includes 5 weight % of Sn, 5 weight% of Zn, and a remainder of Mg.

Furthermore, Mg alloys described with reference to FIG. 4 to FIG. 9 are,particularly, Mg alloys manufactured under the following conditions:

1. Melting and Casting Step (Common Step)

Component elements of each alloy described above are measured in termsof weight %, are melted in an electric resistance furnace that ismaintained at 750° C. in SF₆+CO₂ mixed gas atmosphere, and are cast in amold having a diameter of 52 mm and a length of 100 mm.

2. Case 1

2-1. Homogenization Step

After casting, the test piece is loaded into an electric resistancefurnace maintained at 330° C., is maintained for 24 hours, and iswater-cooled.

2-2. Extrusion Step

After the test piece is loaded into an electric resistance furnace(inside an extruder) maintained at 300° C., a thermometer is attached tothe test piece, and, when the temperature reaches 270° C., the testpiece is instantly extruded at an extrusion ratio of 19:1.

The above method gives a rod-shaped test piece having an initialdiameter of 49.5 mm and, after extrusion, a plate-shaped test piece withcross section of 25×4 mm².

3. Case 2

3-1. Solution Treatment Step

Mg—Sn binary alloy is maintained at 450° C. for 24 hours and iswater-cooled. Mg—Sn—Zn(—Al) alloy is maintained at 330° C. for 18 hours,is temperature-raised to 450° C. for two hours, maintained for 12 hours,and is water-cooled.

3-2. Aging Step

Test pieces are loaded into an electric resistance furnace maintained at200° C. Mg—Sn binary alloy is maintained for 500 hours, andMg—Sn—Zn(—Al) ternary (quaternary) alloy is maintained for 24 hours.Subsequently, both of the alloys are air-cooled.

3-3. Extrusion Step

In the same manner as the process of Case 1, after the test piece isloaded into an electric resistance furnace (inside an extruder)maintained at 300° C., a thermometer is attached to the test piece. Whenthe temperature reaches 270° C., the test piece is instantly extruded atan extrusion ratio of 19:1.

The above method gives a rod-shaped test piece having an initialdiameter of 49.5 mm and, after extrusion, a plate-shaped test piece withcross section of 25×4 mm².

4. Tensile Test

A test piece having ASTM specification gauge length of 25 mm (KSB0801proportional test piece no. 13B) is machined and subjected to a tensiletest under a condition of initial strain rate: 1×10⁻³.

FIG. 4 illustrates engineering stress and engineering strain curves ofMg alloys according to an exemplary embodiment of the present inventionand related-art Mg alloys. It is clear from FIG. 4 that, given the samecomposition, the alloys according to the present embodiment havestresses about 20% better than those of the related-art alloys.

In addition, FIG. 5 illustrates a relationship between the UTS andelongation of Mg alloys according to an exemplary embodiment of thepresent invention and related-art Mg alloys. It is clear from FIG. 5,with regard to alloys of all compositions, both UTS and elongation ofalloys according to the present embodiment are superior to those of therelated-art alloys.

FIG. 6A to FIG. 6C are SEM pictures of Mg alloys at a step aftersolution treatment and aging and before plastic deformation inconnection with manufacturing of Mg alloys of various compositionsaccording to an exemplary embodiment of the present invention. Whenhomogenization is solely performed before plastic deformation accordingto the related-art method, coarse crystallized phases created aftercasting are distributed at grain boundaries. It is clear from FIG. 6A toFIG. 6C that, as a result of aging after solution treatment, smallsecond phases are uniformly distributed inside/outside grains. That is,white portions appearing at grain boundaries in the low-magnificationpictures in FIG. 6A to FIG. 6C are pre-precipitated second phases, andthe formation of uniform second phases inside grains is alsoappreciated.

FIG. 7A and FIG. 7B are scanning electron microscopy (SEM) pictures of aMg alloy manufactured by a related-art method (FIG. 7A) and a SEMpicture of a Mg alloy manufactured according to an exemplary embodimentof the present invention (FIG. 7B). It is appreciated from the picturein the middle of FIG. 7A, a low-magnification picture, that secondphases are concentrated at the right-hand side and are scarcelydistributed on the left-hand side. That is, the size of grains in theregion of a small distribution of second phases is larger than that ofthe region of a large distribution, meaning that the structure hasdifferent grain sizes depending on the amount of distribution of secondphases. In contrast, it is appreciated from FIG. 7B that, compared withFIG. 7A, the second phases are distributed evenly, and the grain size isuniform and small.

FIG. 8A is a picture taken after the extrusion of a Mg alloy accordingto the related-art method, second phases appear as white portions, andit is clear that the distribution is not even because a test piece thathad irregular distribution of second phases before plastic deformationhas been extruded with no modification. In contrast, FIG. 8B is apicture taken after the extrusion of a Mg alloy that has been formed byan exemplary embodiment according to the present invention, and it isclear that, compared with the case of FIG. 8A, small second phases aredistributed evenly.

FIG. 9A and FIG. 9B are TEM pictures of Mg alloys formed by arelated-art method (FIG. 9A) and according to an exemplary embodiment ofthe present invention (FIG. 9B). Similar to the results of SEM picturesdescribed above, it is clear that the Mg alloy according to an exemplaryembodiment of the present invention, compared with the Mg alloy formedby the related-art method, has smaller second phases distributed evenly.

Although a method of manufacturing a Mg alloy, which includes Sn, andadvantageous effects thereof have been described above, those skilled inthe art, to which the present invention pertains, could understand thatthe Mg alloy according to the present invention can be applied similarlywhen Zn, Al, Li, and Pb are included, besides Sn. It is apparent to aperson skilled in the art to which the present invention pertains thatvarious changes and modifications can be made to the aboveconfiguration. Therefore, the scope of the present invention is solelylimited by the accompanying claims.

What is claimed is:
 1. A magnesium alloy comprising: an element selectedfrom the group consisting of more than 0 weight % and 14 weight % orless of Sn, more than 0 weight % and 5 weight % or less of Li, more than0 weight % and 40 weight % or less of Pb, more than 0 weight % and 17weight % or less of Al, and more than 0 weight % and 5 weight % or lessof Zn; and a remainder of Mg, wherein a second phase comprising at leastone selected from the group consisting of Mg₂Sn, Mg₂Zn₃,Mg_(47.2)Zn_(36.9)Al_(16.9), Mg₁₇Al₁₂, α-Mg/βLi phase and Mg₂Pb isformed in the alloy, the second phase comprises precipitation phases,and, among the precipitation phases constituting the second phase,precipitation phases having a size exceeding 10 μm are less than 0.1% ofthe entire precipitation phases.
 2. The magnesium alloy of claim 1,wherein the second phase of the magnesium alloy is uniformly distributedin entire crystal grains.
 3. The magnesium alloy of claim 1, wherein thesize of crystal grains of the magnesium alloy is substantially evenlydistributed.
 4. The magnesium alloy of claim 3, wherein the second phaseis Mg₂Sn phase.
 5. The magnesium alloy of claim 3, wherein the magnesiumalloy is a plastically deformed plate member.
 6. The magnesium alloy ofclaim 3, wherein the plastically deformed plate member is an extrudedplate member.
 7. A method of manufacturing a magnesium alloy, the methodcomprising: melting and casting raw materials comprising an elementselected from the group consisting of more than 0 weight % and 14 weight% or less of Sn, more than 0 weight % and 5 weight % or less of Li, morethan 0 weight % and 40 weight % or less of Pb, more than 0 weight % and17 weight % or less of Al, and more than 0 weight % and 5 weight % orless of Zn and a remainder of Mg; subjecting the cast magnesium alloy tosolution treatment; subjecting the magnesium alloy that has undergonesolution treatment to aging; and plastically deforming the agedmagnesium alloy, wherein a second phase comprising at least one selectedfrom the group consisting of Mg₂Sn, Mg₂Zn₃, Mg_(47.2)Zn_(36.9)Al_(16.9),Mg₁₇Al₁₂, α-Mg/β-Li phase, and Mg₂Pb is formed in the alloy, the secondphase comprises precipitation phases, and, among the precipitationphases constituting the second phase, precipitation phases having a sizeexceeding 10 μm are less than 0.1% of the entire precipitation phases.8. The method of claim 7, wherein the size of crystal grains of themagnesium alloy is substantially evenly distributed.
 9. The method ofclaim 7, wherein the second phase is uniformly distributed in entirecrystal grains.
 10. The method of claim 7, further comprising: after theplastically deforming the aged magnesium alloy, annealing theplastically deformed alloy.
 11. The method of claim 10, wherein theplastic deformation is extrusion.
 12. The method of claim 8, furthercomprising: after the plastically deforming the aged magnesium alloy,annealing the plastically deformed alloy.
 13. The method of claim 12,wherein the plastic deformation is extrusion.
 14. The method of claim 9,further comprising: after the plastically deforming the aged magnesiumalloy, annealing the plastically deformed alloy.
 15. The method of claim14, wherein the plastic deformation is extrusion.